The plant hormone auxin (IAA, indole-3-acetic acid) is a crucial informational molecule that controls many aspects of plant development, including differentiation of particular tissues such as xylem, branching of both shoots and roots, and elongation and tropic responses to light and gravity. In the model plant Arabidopsis thaliana, shy2 mutations fall in the IAA3 gene encoding a member of this family. Study of these mutants has shown that SHY2/IAA3 is an important regulator of auxin-controlled development and also affects responses to light. This proposal describes experiments to determine how SHY2/IAA3 may link light perception and auxin responses, and how it regulates development. The approaches will include analyses of shy2 mutant phenotypes; studies of SHY2/IAA3 protein interactions with other regulatory proteins; studies of SHY2/IAA3 protein modification, stability, and localization in plants; studies of global gene expression patterns in shy2 and other mutants; and characterization of genetic modifiers of a shy2 mutation. These experiments will help us understand how auxin regulates development in a model system, and will in the future allow manipulation of morphology of crop plants in ways beneficial to human health.